The invention relates to a process for catalytic cracking, more specifically a process for treating heavy hydrocarbon oils, as well as an apparatus permitting the performance of this process.
The evolution of the nature of crude oil supplies, as well as the reduction in demand for products refined into fuel oils compared with lighter products of the petroleum type have led the oil refining industry to develop various processes making it possible to valorize natural heavy oils, as well as residual oils such as atmospheric distillation residues or vacuum distillation residues.
With respect to the catalytic cracking processes using such charges, it has been found that the main difficulties are due to the fact that these charges contain organic materials such as asphaltenes, as well as condensed polycyclic aromatic hydrocarbons, which are difficult to crack into lighter molecules to provide high yields of fractions boiling at lower boiling points, but which tend to associate in order to give rise to large coke formations, which are deposited on the catalysts used and reduce their activity. Moreover, the presence in these heavy oils of large quantities of heavy metals, such as, e.g. nickel, vanadium, iron, etc., is considered to be prejudicial, because these metals tend to poison or at least deactivate the zeolite-type catalysts generally used in existing fluid catalytic cracking processes, referred to as F.C.C.
A certain number of essential factors can still make it possible to convert heavy products into lighter fractions with a good selectivity, while keeping coke formation to a minimum.
Among these factors, one of the most important consists of ensuring, with respect to the contacting of the hydrocarbon charge (which is generally preheated and to which is optionally added steam) and the regenerated hot catalyst used in the catalytic cracking unit, a mixture such that the heat transfer between the charge and the catalyst is carried out as rapidly as possible and as regularly as possible. It is also vital for the renewal of the regenerated catalyst that the introduction of the charge into the reaction zone is carried out permanently and efficiently, while in particular avoiding retro-mixing phenomena, which increase the contact times and lead to a significant reduction in the formation of light fractions by increasing the percentage by weight of coke formed on the catalyst grains.
Another important phenomenon, which is also prejudicial to the satisfactory operation of a F.C.C. unit, more particularly intended for the treatment of heavy charges, involves of the difficulty of obtaining a good radial homogeneity of the catalyst, both at the start of the reaction zone and all along the reaction zone.
With regards to the selectivity of the cracking reactions in general terms, it is known that it improves as the flow of fluids approaches a piston-type flow, which is particularly difficult to obtain in conventional F.C.C. units.
The present invention aims at simultaneously obtaining the optimum conditions for achieving the three aforementioned factors, namely the absence of retro-mixing, radial homogeneity and a piston-type flow. This objective can be achieved in a reaction zone traversed by a co-current of charge and catalyst in the falling or downward direction, the catalyst being brought to a well defined fluidization level prior to its contact with the charge.
Among the F.C.C. processes described in the prior art and using a downward co-current reactor in the reaction zone, certain processes are intended for the treatment of conventional charges such as vacuum gas oils. U.S. Pat. No. 2,420,558 describes such a method, but only uses a single regeneration zone of the catalyst. U.S. Pat. No. 2,965,454 describes an apparatus, whose reaction zone is constituted by a plurality of vertical tubes traversed in falling co-currents of charge and catalyst. U.S. Pat. No. 3,835,029 also relates to a falling co-current F.C.C. process, which is only applicable to light charges (entirely vaporizable between 510.degree. and 550.degree. C.), whose introduction takes place in the vapor phase below the introduction of the catalyst into the reaction zone, when the speed of the catalyst is between 9 and 30 m/s.
These methods are not suitable for heavier hydrocarbon charges, because the choice of such a high catalyst speed makes it necessary for the charge to be completely vaporized prior to contact therewith. However, the introduction of a heavy charge in the vaporized state would necessarily be accompanied by an undesired thermal cracking (coke formation).
U.S. Pat. No. 4,385,985 describes an improved downflow catalytic cracking process making it possible to treat heavy charges, which have a boiling point above 560.degree. C. and a Conradson carbon content equal to or higher than 3%. However, the system, consisting of introducing the catalyst from a dense phase in the form of a flow of particles with an uncontrolled apparent density, does not permit a homogeneous distribution of the catalyst with respect to the injection and consequently a complete, fast vaporization of the charge throughout the entire reactor cross-section.
The prior art can also be illustrated by U.S. Pat. No. 4,532,026.
However, none of the aforementioned apparatuses makes it possible to bring about an appropriate heavy hydrocarbon injection into a catalytic cracking reaction zone, while ensuring both a rapid mixing (below preferably 500 milliseconds) of the catalyst and the charge vaporized on contact therewith, an increase in the heat exchange coefficient and a good radial mixing homogeneity over the entire surface of the reactor.
Therefore the present invention relates to a novel F.C.C. process (fluid catalytic cracking) obviating the aforementioned disadvantages and which is more particularly usable for the transformation of heavy hydrocarbon charges. Thus, the charges can either be conventional charges, e.g., having final boiling points of approximately 400.degree. C., such as vacuum gas oils, as well as heavier hydrocarbon oils, such as crude and/or stripped oils, together with vacuum distillation or atmospheric distillation residues. If appropriate, these charges may have had a prior treatment such as, e.g. a hydrotreatment in the presence of cobalt-molybdenum type catalysts. The preferred charges according to the invention are those containing fractions normally boiling up to 700.degree. C. and higher, which can contain high percentages of asphaltene products and which have a Conradson carbon content up to 4% and above. These charges may or may not be diluted by conventional lighter fractions, which can include hydrocarbon fractions which have already undergone the cracking operation, but which are recycled, such as, e.g. light cycle oils (L.C.O.) or heavy cycle oils (H.C.O.). According to the preferred embodiment of the invention, these charges are preheated in a temperature range between 100.degree. and 250.degree. C. prior to their treatment.
In general terms, the invention relates to a fluid catalytic cracking process for a hydrocarbon charge in a reaction zone, which is preferably substantially vertical, in which the charge and the catalyst circulate in top to bottom, co-current manner, the process involving a stage of supplying at least partly regenerated catalyst to the upper end of the reaction zone, a stage of introducing and atomizing the charge in an introduction zone located below the catalyst supply zone to the upper part of the reaction zone, a stage of contacting the catalyst and the charge in the upper part of the reaction zone, a stage of circulating the catalyst and the charge in the reaction zone during which the cracking of the charge takes place under appropriate cracking conditions and the catalyst is at least partly deactivated by depositing coke thereon, a stage of separating and stripping the deactivated catalyst from the cracked hydrocarbon charge in a separation zone at the lower end of the reaction zone, a stage of regenerating at least part of the deactivated catalyst in at least one regeneration zone and a stage of recycling the at least partly regenerated catalyst into a recycling zone upstream of the upper end of the reaction zone, characterized in that
a) during the recycling stage, the at least partly regenerated catalyst is sampled from a dense phase, PA1 b) the stage of supplying catalyst to the upper end of the reaction zone takes place in the presence of at least one fluidization gas and a suspension of catalyst and gas is obtained, PA1 c) the density of the suspension is measured by appropriate measuring means upstream of the charge introduction zone and PA1 d) by adequate means, adjustment and regulation takes place of the flow rate of the fluidization gas upstream of the charge introduction zone under conditions such that the density of the gas-solid suspension formed in this way is between 80 and 500 kg/m.sup.3 prior to its contacting with the charge. PA1 according to a first variant, it is possible to separate the deactivated catalyst from the cracked charge by gravity in the separation zone and then a cyclone separation stage is performed on at least part of the catalyst entrained by the effluent and a cracking effluent is recovered; PA1 according to a second variant, it is possible to separate in the separation zone the deactivated charge from the cracked charge by carrying out, at the outlet from the reaction zone, a cyclone separation stage relative to all the catalyst and effluent and a cracking effluent is recovered. PA1 a) at least one ionizing radiation source and at least one detector of ionizing radiation, the source and detector being positioned at a level which is downstream, in the displacement direction of the solid, of the introduction level of the solid, so as to make it possible to measure the mean density of the moving solid, according to at least one predetermined direction substantially perpendicular to the displacement direction of the charge or in at least one given zone between the source and the receiver, PA1 b) means able to regulate a fluidization fluid flow valve, PA1 c) means, which are, e.g., in the form of programmable calculations, which make it possible on the basis of the acquisition of the value of the density, in at least one predetermined direction or in at least one given zone and at a given time, to carry out the cracking of the hydrocarbon charge under predetermined, stable conditions, by comparison with reference values of the various parameters, which are, e.g., stored in the programmable calculating means and by continuously or periodically adjusting the fluidization fluid flow rate on the basis of the means b). PA1 M.sub.V density (kg/m.sup.3) PA1 g gravitational acceleration (m/s.sup.2) PA1 h distance separating the two pressure transducers (m) PA1 .DELTA.P pressure drop (Pa).
The aforementioned operating conditions are very close to those recommended in configurations with a riser. Therefore, the heat exchange coefficient between the catalyst and the heavy charge is increased and the distribution of catalyst in accordance with a radial plane is more homogeneous. This leads to a faster vaporization of the charge, which contributes to a better petroleum fraction selectivity, which is increased if retro-mixing phenomena are minimized. Despite the type of heavy charges treated, there is a reduced coke deposition on the catalyst.
The fluidization conditions can be such that the velocity of the catalyst prior to its contacting with the charge is between approximately 2 and approximately 8 m/s. According to a particularly advantageous feature, prior to its contact with the charge the catalyst can have an apparent density in the solid-gas suspension of 200 to 300 kg/m.sup.3 and a velocity of 3 to 5 m/s.
Under these conditions the best results are obtained, particularly with regard to the heat exchange coefficient between the catalyst and the heavy charge, which is consequently more rapidly vaporized.
It is also preferable to regenerate the catalyst, deactivated by the coke deposited during the cracking reaction, in two regeneration zones. This catalyst is first fed into a first regeneration zone, where it is partly regenerated, then fed into a second regeneration zone at a higher temperature generally between 650.degree. and 900.degree. C. and preferably between 750.degree. and 800.degree. C. and the thus regenerated catalyst is recycled at a temperature of, e.g., 800.degree. C. to the cracking reaction zone.
According to a first embodiment of the process, the recycling zone is rising overall and the rising speed of the catalyst is generally 5 to 20 m/s as a result of appropriate fluidization means.
According to another advantageous feature of the invention, the recycling zone of the catalyst can comprise, downstream of the rising zone, a dense fluidized bed storage zone where the regenerated catalyst is made to flow out. It is sampled from the dense bed and is then made to circulate from the storage zone to the upper part of the reaction zone under conditions such that its apparent density in the suspension is 80 to 500 kg/m.sup.3 prior to contacting with the atomized charge, the fluidization of the catalyst being ensured by at least one fluidization means located at an outlet of a tube from the storage zone upstream of the cracking reaction zone. Under these fluidization conditions, the velocity of the catalyst can reach 2 to 8 m/s.
According to a second embodiment of the process, the recycling zone can be downward and the catalyst is advantageously sampled from the dense fluidized bed of the second regenerator and flows towards the reaction zone under the aforementioned speed and apparent density conditions as a result of a fluidization means preferably located at an outlet of the tube leading to the cracking reaction zone.
The separation of the catalyst and the cracking effluent can, e.g., be carried out in two ways:
The invention also relates to an apparatus in particular for performing the process. More specifically, it comprises a preferably substantially vertical, elongated tubular reactor or dropper appropriate for the cracking of the charge in catalytic, co-current top to bottom manner and having an upper end and a lower end, the upper end having means for the introduction and atomization of the charge, means for the introduction of the catalyst located upstream of the charge introduction and atomization means, at least one separating enclosure connected to the lower end of the reactor containing primary separation means of the deactivated catalytic particles from the cracked charge, secondary separation means by steam stripping of most of the catalyst entrained by the cracked charge located in the lower part of the separating enclosure and at least one cracked charge or reaction effluent recovery means, the apparatus also having at least one deactivated catalyst regenerator having an inlet connected to the lower part of the separating enclosure and means for recycling the at least partly regenerated catalyst located between a lower outlet of the regenerator and the upper end of the reactor, characterized in that the recycling means (26b,21) have in combination at least one means (30) for fluidizing the catalyst by an appropriate gas for bringing the catalyst to an appropriate apparent density upstream of the charge introduction and atomization means (5) and checking and regulating means (28) suitable for cooperating with the fluidization means.
The length of the reactor can be 6 to 20 times its diameter starting from the charge introduction point and is preferably 10 to 15 times. It can be defined in such a way as to determine the residence time required by the desired degree of conversion and without modifying the elevation of the unit.
According to another feature, the charge introduction means are located at a distance from the fluidization means corresponding to 1 to 5 and advantageously 2 to 3 times the reactor diameter.
According to an apparatus variant, the recycling means can comprise a substantially vertical, elongated column, whose lower end is connected to the lower part of the second regenerator by a connecting tube and whose upper end is connected to the upper end of the reactor, the lower end of the column having at least one gas fluidization member cooperating with the regulating and checking means.
According to a more advantageous variant, the recycling means incorporate a substantially vertical, elongated column, whose lower end is connected to the lower part of the second regenerator by a connecting tube, whose upper end is connected to a storage enclosure able to operate in a dense fluidized bed as a result of secondary fluidization means located in the lower part of the enclosure, the lower end of the column having at least one first gas fluidization member, the storage enclosure having in its lower part an outlet tube connected to the upper end of the reactor, which has at least one gas fluidization means cooperating with the checking and regulating means.
The introduction means are known injectors arranged symmetrically in the injection section and which are conventionally directed towards the axis of the reactor under an angle advantageously between 20.degree. and 50.degree. with respect to the vertical. They are able to supply liquid droplets having a diameter generally between 50 and 100 micrometers and which have a velocity of, e.g., 50 to 100 ms, as a result of the medium pressure atomization steam normally introduced in a proportion of 2 to 10% by weight, based on the charge and preferably 4 to 6% by weight.
According to the main feature of the invention, the regulating and checking means of the fluidization means are described in French patent application 89/15600 and comprise:
The use of an ionizing radiation source and a detector for ionizing radiation positioned in the same plane, on either side of the reaction zone and at a chosen level, preferably in the immediate vicinity and upstream of the charge injectors makes it possible by measuring the variation of the absorption of (said variation being directly linked with the variation of the density of the solid at the location of the measurement) to check and, e.g., maintain the density between two previously chosen extreme values. It is possible to carry out a series of measurements in a series of directions in the same plane and thus obtain a distribution cartography of the density of the solid in the measurement plane. Such a cartography can be carried out with the same mobile assembly, constituted by the source and detector, or with the aid of several sources and several detectors. The ionizing radiation source can be positioned outside the reaction zone or inside the downward reaction zone, e.g., in the vicinity of its periphery. The same applies regarding the radiation detector. Preferably, the source and detector are positioned outside the reaction zone.
It is possible to use any emitter of ionizing rays having an adequate emission power estimated as a function of the characteristics of the equipment used. The radiation source used is normally an alpha, beta, gamma or X radiation source, or a neutron source. Most frequently use is made of a gamma radiation source, such as, e.g., a conventional sealed cesium 137 or cobalt 60 source. The power of the sources used is approximately 1 to 100 Curies. The radiation detector is usually a conventional ionizing radiation-sensitive detector, such as, e.g., a photomultiplier or an ionization chamber.
The precise position or positions of the ionizing radiation source or sources and the corresponding detector or detectors, e.g., between the introduction zone of the solid incorporating catalytic particles and the introduction zone of the hydrocarbon charge, can easily be determined, particularly as a function of the size of the reaction zone and the average density chosen for the entrained fluidized bed under the conditions used.
According to an apparent density determination method, it is possible to use a pair of pressure transducers (P1,P2), which are spaced, e.g., by the distance of the diameter of the reactor and are positioned in accordance with the generatrix of the cylindrical cracking reactor between the fluidization means and the charge injector.
The recorded pressure differences are linked by the static pressure drop equation with the density of the suspension by the relation EQU .DELTA.P=P.sub.2 -P.sub.1 =M.sub.v gh
The pressure drop is compared by a microcomputer with a reference pressure drop and the microcomputer supplies, as a result of this comparison, a signal permitting an at least partial opening or closing of the fluidization means.
Within the scope of the present invention no mention is made of the type of catalysts used, or the various fluidization fluids, which are of a conventional nature and are well known in the art. Preferably, use is made of steam and zeolitic catalysts, at least one inert gas or gaseous hydrocarbons as the fluidization fluid, both in the rising column and in the falling column prior to the reaction zone.